Box culvert base slab

[Tstruct]

Hello,
I have a question regarding box culvert. Please have a look at the attached image.

I have to design a box culvert for the condition shown in "1". This is the natural ground condition (not excavated). Basically it is an existing path of water flow.
The issue is where should I place my base slab of the box culvert? According to my building design knowledge, it should be around 5 feet below the water bed to get some hard strata and avoid settlement.
Whereas, I have never seen any such construction on internet or real life (very limited exposure). What I have seen is top level of base slab is in alignment of water course.
Please advice me on this matter.

 

[JoelTXCive]

I think you need to determine your desired flowline of the channel; and then work with a geotech to see if improvements need to be made to the soil.

 

[DoubleStud]

I have done both but you need to do manning equation calculation to make sure your box culvert is large enough and you have enough room to support the truck load above. Remember, sometimes you have worse condition if you barely have any soil on the top because the truck impact load will be huge. I have done 3 because the county or the client wants natural bottom for aesthetic reason or for fish migration. Sometimes scenario 3 is deep enough you do not need a cut off wall at the headwall. For scenario 2, you will definitely need cut off wall at the headwalls to prevent scour.

 

[Tstruct]

@DoubleStud In case of scenario "2", what is the approach behind putting the base slab directly on the existing soil instead of a competent strata deep down? Did you do any soil improvement? usually on internet I have seen base slab resting directly on natural ground level (with zero or minimal excavation). I am stuck here, couldn't find literature on it. Can you please help?

 

[BridgeSmith]

Generally, the invert (top of bottom slab) is at flowline (stable streambed elevation) . Burying it deeper requires a taller box, since the clear height is dictated by the hydraulics or other clearance considerations, and the clear height is measured from the flowline, regardless of where the bottom slab is. It's typically (nearly always) less expensive to do subexcavation and ground improvement than to fabricate larger box sections.

The only time we sink the bottom slab below flowline is when a natural bottom (natural streambed material) is specified through the box for fish passage, etc.